1. Field of the Invention
The present invention relates to a method and apparatus for performing welding operations and, more precisely, to a technique for detecting and controlling the indentation of a weld during resistance welding operations.
2. Description of the Prior Art
Resistance welding refers to a type of welding in which separate parts are welded together by heat generated by the passage of an electrical (welding) current through a welding area occurring in those parts. The amount of heat generated thereby is governed by the electrical resistance of these parts to the passage of an electrical current through the welding area and the magnitude of this current. During the welding process, mechanical pressure is used to force the parts together. In resistance spot welding, which is by far the most common type of resistance welding, a weld is formed by localized welding and subsequent coalescing of a small volume of material located at two abutting faying surfaces due to the heating caused by the passage of the welding current through these surfaces. A detailed discussion of the resistance welding process may be found in Resistance Welding Manual, (Third Edition) published by the Resistance Welder Manufactures' Association.
There is a direct relationship between electrode indentation measured after the welding process has been completed and the strength of the weld, as documented in K. C. Wu, "Electrode Indentation Criterion for Resistance Spot Welding," Welding Journal, October 1968, pages 472-S to 478-S (hereinafter referred to as the "Wu paper"). Many attempts have been made to measure and/or control the resistance spot welding process by measuring any change in the distance between the tips of two opposing electrodes, i.e. inter-electrode displacement, while two (or more) materials situated therebetween are being welded to each other. See, for example, the following U.S. Patents which disclose apparatus which measures inter-electrode displacement: U.S. Pat. Nos. 4,419,558 (Stiebel); 4,542,277 (Cecil); 4,296,304 (Defourney); and 4,447,700 (Cohen).
Indentation is defined to be a depression that is produced in an external surface of a material being welded and caused primarily by a welding electrode forging the surface of the material in the spot weld area or the nugget region, i.e. the area where the material was liquid during the formation of the weld nugget. The external surface of the material adjacent to the nugget on one or more sides becomes indented or depressed due to the reduced hot strength of the material which supports the electrodes. As such, the electrodes exert a forging force while the weld nugget is being formed.
Measurements of inter-electrode displacement resulting from growth of the material between the electrodes during the time welding current is flowing has led researchers to believe that most of the indentation occurs while the nugget is solidifying, i.e. after the welding current has been turned off. U.S. Pat. No. 4,419,558 (Stiebel), hereinafter referred to as the '558 patent, at Column 2, line 46 indicates that the amount of indentation needed to control the welding process, i.e. to ensure that welds having sufficient strength are being formed, is about 0.001 inches (approximately 0.0025 centimeters) for sheet metal, which is typically about 0.040 inches (approximately 0.10 centimeters) thick. Thus, the desired indentation as taught by the '558 patent is about 2.5% of the total sheet thickness. However, this value is not consistent with published data. For example, the Wu paper indicates that an indentation of 5 to 10 percent of the thickness of a sheet is the most desirable amount of indentation to produce welds having sufficient strength. Clearly, five percent of 0.040 inches (approximately 0.10 centimeters) is 0.002 inches (approximately 0.0051 centimeters) and 10 percent of 0.040 inches is 0.004 inches both of which values are well in excess of the values disclosed in the '558 patent. Additionally, thicker sheet materials would have proportionately larger indentations. Unfortunately, all such indentations are much larger than the measured values obtained form inter-electrode displacement measurements.
The small measurement obtained by measuring inter-electrode displacement occurs because such a measurement fails to take account of thermal expansion of the material that occurs during the welding process. Specifically, in measuring inter-electrode displacement, the measured value contains a portion resulting from thermal expansion and another portion resulting from indentation. Since thermal expansion occurs in a direction opposite to that for indentation, the material growth resulting from expansion cancels out (masks) a portion of the material shrinkage resulting from indentation thereby causing the overall indentation to change much less than would be expected. While the Wu paper shows that a direct relationship exists between weld strength and indentation, the applicant is aware of no teachings in the art that correlate weld strength to material expansion. Now, for some combinations of materials, electrode shape and welding conditions, expansion may be equal to or considerably larger than indentation. Consequently, indentation, measured through inter-electrode displacement, may be zero due to expansion when, in fact, indentation has actually occurred. As a result, measurements of inter-electrode displacement made during a welding operation can not generally be used to accurately assess weld strength.
In this regard, see A. Stiebel et al, "Monitoring and Control of Spot Weld Operations," Conference Proceedings of the 1986 Sheet Metal Welding Conference II, Detroit, Mich., Oct. 27-29, 1986, pages 1-17 (hereinafter referred to as the "Stiebel paper"). The Stiebel paper does recognize that indentation is indicative of weld strength. Unfortunately, this paper discloses a system for measuring indentation based on measurements of force induced inter-electrode displacement. As noted above, inter-electrode displacement resulting from expansion greatly masks any indentation occurring during the formation of the weld. With such a system, indentation can only be accurately measured after the weld has been made and, more specifically, after the material has contracted back to its original thickness. Consequently, the system disclosed in the Stiebel paper can not accurately measure indentation occurring during weld formation and much less control the indentation as it occurs--thereby exhibiting problems typical to all systems known in the art that rely on using measurements of inter-electrode displacement.
Therefore, a need exists in the art for apparatus, and accompanying methods for use therein, that accurately measures the indentation that occurs during a welding operation. A need also exists in the art for apparatus and accompanying methods to accurately control a resistance spot welder, based upon the measured indentation, in order to ensure that substantially all the welds made thereby have sufficient strength.